While not as fast as open-wheelFormula One cars, LMPs are the fastest closed-wheel racing cars used in circuit racing. Le Mans Prototypes are considered a class above production-based grand tourer cars, which compete alongside them in sports car racing.

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Le Mans Prototypes have used various names depending on the series in which they compete. The FIA's equivalent cars were referred to as Sports Racers (SR) or Sports Racing Prototypes (SRP). The American IMSA GT Championship termed their cars World Sports Cars' (WSC), while the short-lived United States Road Racing Championship used the classic Can-Am (CA) name for their prototypes. Since 2004, most series have switched to referring to these cars as Le Mans Prototypes. The American Le Mans Series, the successor to the IMSA GT Championship and the predecessor of the TUDOR United SportsCar Championship, officially referred to the cars simply as Prototypes (P1, P2, or PC). An LMP is commonly referred to as a Le Mans car in the media.[2][3]

The first use of what would become Le Mans Prototypes was at the 1992 24 Hours of Le Mans. In an attempt to increase the number of entrants beyond the small field of Group C competitors that the World Sportscar Championship had to offer, older Porsche 962s were allowed entry in Category 3. To further increase the size of the field, small open-cockpit race cars using production road car engines which were raced in small national championships, were allowed in Category 4. Only three cars (a Debora-Alfa Romeo, a Ren-Car Peugeot and a WR-Peugeot) were entered, with all failing to run more than a few hours.

However at the end of 1992, the World Sportscar Championship as well as the All Japan Sports Prototype Championship collapsed, leaving the expensive Group C prototypes little competition outside of Le Mans. With Group C being phased out, the ACO chose to allow production-based race cars to enter for the first time in many years, while at the same time creating the Le Mans Prototype (LMP) class. The cars continued to use the same formula as they had in 1992.

Later, ACO announced their intentions to completely replace the Group C cars with Le Mans Prototypes in 1994. However, LMP1 cars this year were just ex-GroupC cars (Toyota 94 C-V, Courage C32, Kremer K8, Porsche 962 GTI, ALD C289 and Alpa LM) Two classes were created, with LMP1s running large displacement custom-built engines that were usually turbocharged, and LMP2s using the smaller displacement production-based engines. Both classes were required to have open cockpits. At the same time, the IMSA GT Championship announced the end of their closed cockpit GTP and Lights classes, deciding as well to replace them with a single open-cockpit class of World Sports Cars equivalent to LMP1.

This formula continued up to 1996, with many manufacturers embracing the LMP and WSC classes, including Ferrari, Porsche, and Mazda. In 1997, the first European series based around Le Mans Prototypes was launched, known as the "International Sports Racing Series". Using classes similar to LMP1/WSC and LMP2, these cars were known as "SR1" and "SR2" by the FIA. 1998 saw the creation of another series of Le Mans Prototypes, with the new United States Road Racing Championship (USRRC) attempting to break away from the IMSA GT Championship. To differ from IMSA'S WSC class, the USRRC named their open-cockpit prototypes "Can-Am" in an attempt to resurrect the sportscar championship of the 1970s. However the USRRC collapsed before the end of 1999, with the series becoming the Rolex Sports Car Series who chose to use the FIA's SR1 and SR2 formula instead.

1998 saw a great expansion for the ACO's LMP classes. Following the cancellation of the IMSA GT Championship at the end of 1998, the ACO allowed for the creation of the American Le Mans Series. This series used the same class structure as the 24 Hours of Le Mans, meaning it was the first championship to use the LMP name. At the same time, the ACO greatly altered their LMP classes. The smaller LMP2 class were briefly eliminated, while a new class of closed-cockpit prototypes were allowed in, known as "LMGTP" (Le Mans grand touring prototype). These cars were evolutions of production-based road cars that the ACO considered too advanced and too fast to fall under the GT class regulations, forcing the ACO to promote them to prototypes.

In 2000, changes were made to the LMP regulations, as the ACO once again split the open-cockpit LMP class. The two new classes became known as "LMP900" and "LMP675", with the numbers denoting the minimum weight requirements (in kilograms) for each class. The LMP900s were to be more powerful and faster in top speed, but also heavier and more cumbersome. The LMP675s were to be smaller and more nimble, yet lack the top speed of the larger class. Both classes were intended to be able to compete for overall wins. Audi, Chrysler, Cadillac, and Panoz opted to use the LMP900 formula, while MG were the only major manufacturer to attempt the LMP675 class. The LMGTP class also continued, with Bentley being the only manufacturer to build a closed-cockpit prototype after the regulation changes in 2000.

Outside of Le Mans, the FIA's SR classes suffered from these rule changes. The SR2 class no longer aligned perfectly with the new LMP675 class, with the more powerful and durable racing engines that were allowed there. The SR1 and LMP900 classes also did not use the same rules, although engines were mostly similar. This meant that teams competing in the newly renamed FIA Sportscar Championship required modifications to their cars to be able to compete at Le Mans and in the new European Le Mans Series (ELMS), a second series split from the American Le Mans Series. With FIA Sportscar Championship teams unwilling to modify their cars to run in the ELMS, that series was canceled due to lack of participants. However the demand to race at Le Mans eventually forced the FIA Sportscar Championship itself to be canceled in 2003, with most competitors choosing to comply with the ACO's regulations instead of the FIA's. With the Rolex Sports Car Series also abandoning their SR classes at the end of 2003 for their own unique Daytona Prototypes, this meant that the ACO LMPs were the only open-cockpit prototypes left.

The dominant entry in the short-lived LMP675 class, the MG-Lola EX257.

With the prototype classes now unified under the ACO's rules, the class structure was once again reorganized. The LMP675 class was considered a failure, due to the small engines lacking the reliability necessary to compete for overall wins, regardless of any advantage they had with cornering and weight. The LMGTP class was also considered redundant since the cars had only minor rule differences from LMP900s. Thus, the classes were changed to LMP1 and LMP2, with the top class once again being larger and more powerful. However the smaller LMP2 class was now intended solely for privateers, with major manufacturers encouraged to move to LMP1. This meant LMP2s were no longer meant to run for overall race wins. Since the LMGTP class was eliminated, both LMP1 and LMP2 were allowed to have either open- or closed-cockpit designs. These new rules also added increased safety requirements, including larger rollover hoops and aerodynamic plates attached to the rear of the cars in order to prevent prototypes from becoming airborne in accidents.

The LMP1 and LMP2 classes continue to be used at the 24 Hours of Le Mans and in the newer championships that were created by the ACO: the Le Mans Series in 2004 and the Japan Le Mans Challenge in 2006. In 2004, the ACO renamed LMP900 back to LMP1, and with this they limited the rear wing to 1.9m and reduced the fuel tank capacity from 90L to 80L. This was done in an effort to promote new hybrid LMP1 cars while putting more restrictions on the aging LMP900 cars like the Audi R8. New safety measures were also brought in, and prototypes were now required to have two rollover bars instead of one. The final year that LMP900 chassis were allowed to be entered was 2006.

Until 2016, LMP2 allows both open and closed-cockpit designs (closed cars must have a windscreen, a roof, and doors on each side), while only closed-cockpit design is allowed for LMP1 as it is much faster and for safety. As of 2011, all cars must have fins on the rear bodywork to prevent them from rolling over in the air during crashes. Although a passenger seat is not used, cars must be designed to carry two people. The empty area of the cockpit is usually used to hold electronic devices and cooling equipment. Only production-based engines were allowed in LMP2 with diesel engines permitted from 2013 onwards.

From 2017, in order to limit the costs, FIA will introduce a new set of regulations, which will be locked in through 2020, aiming a significant power increase, to the range of 150 horsepower (which is expected to lead to a four-second decrease in lap time at Le Mans). Gibson Technology will be the exclusive engine supplier, producing a four-litre normally-aspirated V8.[5]

LMP3 is an entry-level prototype class intended for introducing young drivers and new teams to endurance racing before they progress to the higher classes of prototype racing, LMP2 and ultimately LMP1.[8] LMP3 uses closed-cockpit chassis, which can be built by any constructor, powered by a 5-litre normally-aspiratedV8Nissan engine.[9] The class is being used since 2015. In 2017, the International Motor Sports Association uses the LMP3 as their Prototype Challenge race car, this allows the vehicle to race in either series.[10]